Today, the volume or number of the information data reaches to astronomical figures, therefore we are in need of more capable and efficient data communications technique.
Therefore, many methods or technique have been proposed for example, use of very high or ultra high frequency waves, many kind of multiplex communications, and various kinds of modulation techniques.
None the less, the frequency band which is assigned for specified communications is very strictly limited, for example, each public telephone line has a band-pass filter the upper cutoff frequency of which is about 3,000 Hz, therefore we can not use carrier waves the frequency of which is than 3,000 Hz.
Therefore, the maximum speed of telecommunications through a telephone line was usually limited not more than 1,200 baud.
For this reason, the transmission of telefacsimile any small figure through the public telephone line using a simple device requires a lot of time. Of course high speed telefacsimile machines are provided, but these are very intricate and expensive, because it is necessary to use a kind of complicated electronic data processing machines for constriction of information.